Efforts to develop electrochemical cells having PEO based solid electrolyte systems have continued since about 1973. (M. B. Armand, Fast Ion Transport in Solids, North Holland, Amsterdam, p665, (1973); D. E. Fenton et al., Polymer, 14, 589 (1973)). The main advantages of such a cell system are multifold: (1) very high energy density; (2) potential for excellent electrolyte stability; (3) the ability to be configured in nearly any shape since it contains no liquid; (4) the opportunity to be very inexpensive; (5) inherent safety characteristics; and (6) an expansive market if successfully developed. Up to now the key impediment to the successful development of such a polymer cell for room temperature operation is the low ionic conductivity of the solid polymer electrolyte. A major effort to develop the solid polymer electrolyte (“SPE”) system is being carried out by Hydro Quebec and 3M under contract to the United States Advanced Battery Consortium (USABC) for electric vehicle applications. The batteries developed in this effort are operated at approximately 60° C. to 80° C. (140° F. to 176° F.), and achieve about 800 cycles (M. Gauthier et al., J. Power Sources, 54, 163 (1995)). All attempts in this program to successfully develop a room temperature SPE based battery were unsuccessful because of the low ionic conductivity at room temperature of PEO based electrolyte using the lithium trifluoromethane sulfonyl imide [LiN(CF3SO3), LiTFSI] salt (“TFSI”). Based on examination and evaluation of the various solid electrolytes developed to date (L. A. Dominey et al., Electrochim. Acta, 37, 1551 (1992); F. Alloin et al., Solid State Ionics, 60, 3 (1993)), it is quite apparent that PEO based polymer or derivative thereof appears to be the most promising.
One type of PEO investigated thoroughly is the high molecular weight (about 4 million) linear variety, which forms relatively strong, free-standing films at room temperature. Its strength is derived from a semicrystalline microstructure. Lithium ion transport in such materials depends on the complexation of lithium ions by the oxygen atoms in oxyethylene units in the polymer chains. High molecular weight PEO doped with the lithium salt LiN(SO2CF3)2, LiTFSI, has an optimum conductivity of 10−5 S/cm at 80° C. (176° F.) (S. Kohama et al., J. Appl. Polym. Sci., 21, 863 (1977)). Many lithium salt complexes of PEO at room temperature are predominantly crystalline until a melting point of 68° C. (154.5° F.) leading to very poor ionic conductivities of approximately 10−7 S/cm. The improved conductivities using the TFSI salt are due to the plasticizing effect of the anion which substantially reduces the crystallinity of the PEO complex at room temperature. It is important to note that only the amorphous PEO electrolyte is ionically conductive.
Substantial research effort has been devoted to lowering the operating temperature of SPE to the ambient region. To solve this problem, alkyl phthalates and poly(ethylene glycol) dialkyl ether with low molecular weight have been used as plasticizing additives for SPE to reduce the crystalline region and increase the mobility of the SPE molecular chain at ambient temperature. Low molecular poly(ethylene oxide-dialkyl ether compounds) can contribute to increased room temperature ionic conductivity of SPE, but they still have a crystallization problem which decreases the ionic conductivity at certain temperatures. Another approach to attempt to improve the ionic conductivity at ambient temperature was to synthesize a highly branched PEO to decrease the crystalline tendency of PEO main chain and to increase the chain mobility regarding lithium ion transport such as hyper-branched SPE (Z. Wang et al., J. Electrochem. Soc., 146(6), 2209 (1999)), and comb-like SPEs (J. S. Gnanaraj, R. N. Karekar et al., Polymer, 38(14) 3709 (1997)). However, the ionic conductivity of such highly branched PEO is still low at ambient temperature. All of these efforts were intended to create amorphous polymer near ambient temperature.
To apply a SPE to a real practical electrochemical cell system, adequate mechanical strength is required. Simple crystalline PEO may meet that requirement, but most of modified PEO based SPEs are not strong enough for real cell applications. Crosslinked SPEs were developed as a solution, but the crosslinking reaction restricts polymer chain mobility that is needed for lithium ion transport. (U.S. Pat. No. 4,908,283 to Takahashi, U.S. Pat. No. 4,830,939 to Lee, U.S. Pat. No. 5,037,712 to Shackle and U.S. Pat. No. 3,734,876 to Chu). More advanced systems are the interpenetrating network (“IPN”) type SPE that consist of crosslinked polymers and an ionic conducting phase which is mostly low molecular weight PEO base compounds. The ionic conductivity of such systems, however, still depends on the flexibility of poly(alkylene oxide) which has a temperature dependency on its mobility, as well as on its mechanical strength which is not obvious. Most prior patents have disclosed the use of volatile solvents to dissolve the PEO compounds and metal salts. The use of volatile solvents to make the SPEs increase the processing steps such as evaporation and recovery, increase costs of manufacture, and may pose serious environmental and safety issues. U.S. Pat. No. 5,112,512 to Nakamura discloses crosslinking PEO crosslinking agent to siloxane with a PEO side chain which has a reactive unsaturated bond. This crosslinking approach results in a significantly reduced flexibility of siloxane with PEO polymer. The present invention is distinguished in that the siloxane is captured inside the network with no chemical bonds to the PEO crosslinking agent, greatly enhancing flexibility.
Accordingly, the present inventors have developed a new type of IPN polymer electrolyte having PEO grafted onto polysiloxanes as an ion conducting phase and a porous support to overcome the above-mentioned problems such as low room temperature ionic conductivity, chemical and electrochemical stability, as well as safety. The PEO grafted polysiloxanes are liquid compounds, electrochemically stable and have low glass transition temperature with little or no crystallization problems. Notably the present invention does not include any volatile solvent in the polymer electrolyte preparation.